The present invention relates to variable displacement hydraulic devices, and more particularly, to neutral centering mechanisms for use in returning the controls of such a device from either a first or second operating position, back to a neutral position.
Hydrostatic transmissions typically include variable displacement pump units, which include a fluid displacement mechanism, and some means operable to vary the displacement of the displacement mechanism in response to movement (typically rotation) of an input control member or control shaft. The present invention could also be used with a variable displacement motor unit, although the invention is especially adapted for use with a pump unit, and will be described in connection therewith.
Such variable displacement pump units are typically of the "over-center" type, i.e., the displacement mechanism may be displaced from a neutral position to either a first operating position (e.g., forward propel) or in the opposite direction to a second operating position (e.g., reverse propel). In such devices, it has proven to be very difficult to design and manufacture commercially acceptable control devices, by means of which the control shaft of the pump may be returned from either the first or second operating position to an "absolute neutral" position, i.e., a position of the control shaft corresponding to a 0 displacement of the pump. On a vehicle in which the hydrostatic pump is part of the vehicle propulsion system, failure to find neutral may result in the pump still displacing a small quantity of fluid, and the vehicle "creeping" at a low speed, even when the operator has moved the control linkage to what is apparently the neutral position.
As is well known to those skilled in the art, one of the primary difficulties in designing and manufacturing such a neutral centering mechanism is that, typically, the ability of the mechanism to return the control shaft to absolute neutral is dependent upon the ability to maintain very close manufacturing tolerances on one or more of the parts of the mechanism.
An attempt to overcome the problem of manufacturing tolerances, making it difficult to find absolute neutral, in a repeatable and predictable fashion, is set forth in U.S. Pat. No. 4,934,252, for a "VARIABLE DISPLACEMENT PUMP OR MOTOR AND NEUTRAL CENTERING MECHANISM THEREFOR", assigned to the assignee of the present invention.
Although the device successfully eliminated the tolerance sensitivity of the neutral centering mechanism, its commercial success was limited by its cost and complexity. Furthermore, the mechanism of the cited reference would seem inherently to require that it be located on the control shaft, external to the pump, thus permitting the mechanism to be subjected to corrosion and damage, and making it difficult to keep the mechanism properly lubricated.